1. Field of the Invention
The present invention relates to a continuously variable transmission for transmitting power from an output shaft of a drive unit to an input shaft of a centrifugal clutch in which a change gear ratio is controlled by a control unit.
2. Description of Related Art
As shown in FIGS. 1 and 2, for example, power in a power unit 900 of a scooter-type motorcycle 1000 is transmitted in the following order: an engine 100 as a drive unit; a continuously variable transmission 200; a centrifugal clutch 300; a reduction gear 400; and a driving wheel 500. As shown in FIG. 3, centrifugal clutch 300 is mounted between a hollow central axis 12 (secondary or input shaft of centrifugal clutch 300) of a secondary sheave 14 of continuously variable transmission 200 and a final gear shaft 401 (output shaft of centrifugal clutch 300) extended through central axis 12. As shown in FIGS. 3 and 4, centrifugal clutch 300 includes a clutch plate 301, a clutch shoe 302, a clutch spring 303 and a clutch housing 304 (clutch outer).
As shown in FIG. 3, clutch plate 301 is fixed to central axis 12 of secondary sheave 14 of continuously variable transmission 200. Pin 305 for assembling clutch shoe 302 is provided in a protruding manner in clutch plate 301. In the example of FIG. 3, three pins 305 are attached at regular intervals in a circumferential direction. As shown in FIG. 4, one end of clutch shoe 302 is attached to pin 305 for turning, which is attached to clutch plate 301. Clutch spring 303 couples one end of clutch shoe 302 with the other end of an adjacent clutch shoe 302 in the circumferential direction. Clutch spring 303 exerts an elastic reaction force for constantly attracting one end and the other end of the adjacent clutch shoe 302. Clutch housing 304 is a bowl-shaped member attached to final gear shaft 401, which extends through the hollow central axis 12 of secondary sheave 14, and covers an assembly 310 of clutch shoe 302.
When engine 100 is at a stop, as shown in FIG. 4, assembly 310 of clutch shoe 302 is contracted as a whole by the elastic reaction force of clutch springs 303, and clutch shoes 302 and clutch housing 304 are not in contact with each other. When secondary sheave 14 starts rotating at startup of engine 100, as shown in FIG. 5, assembly 310 of clutch shoe 302 expands as a whole with its centrifugal force against the elastic reaction force of clutch spring 303, and clutch shoes 302 come in contact with clutch housing 304. Then, after a stalling state in which clutch shoe 302 and clutch housing 304 transmit torque while sliding, clutch shoes 302 and clutch housing 304 reach a connected state by frictional force acting between clutch shoes 302 and clutch housing 304. Torque in response to the frictional force of clutch shoes 302 and clutch housing 304 is transmitted to output shaft 401. JP-A-2006-71096, for example, discloses such a centrifugal clutch.
A change gear ratio of continuously variable transmission 200 is controlled by a control unit 600, as shown in FIG. 2. Control unit 600 is provided with a database (gear ratio map) for setting in advance target change gear ratios to be a control target based on vehicle speed, engine rotational speed, throttle opening, and the like in various driving modes while a vehicle is running. Control unit 600 follows the gear ratio map to set the target change gear ratio becoming a control target based on information such as actual vehicle speed and throttle opening.
A greater force is needed at startup. Therefore, continuously variable transmission 200 is usually controlled in a predetermined LOW ratio at startup. On the contrary, a startup ratio memorizing means for storing preset change gear ratios at startup according to various throttle openings is provided in JP-A-3194641. Once an engine speed passes a preset value in an upward direction, a startup ratio memorizing means retrieves the change gear ratio at startup that corresponds to the throttle opening, and the change gear ratio of the continuously variable transmission is then adjusted to be the readout change gear ratio.
The same patent document discloses that the target change gear ratio of the continuously variable transmission is set according to the throttle opening at startup, that is, that the target change gear ratio is controlled closer to a TOP side than a maximum LOW ratio according to the throttle opening at startup. Also, according to the same patent document, “because a startup ratio is set in the early stage when the engine speed starts increasing, and because a pulley 6b of a continuously variable transmission 3 is driven, as shown in a dotted line in FIG. 5(b), a smooth start can be performed without the excessive rotational speed of the engine, occurrence of undershooting due to excessive suppression control of the excessive rotational speed of the engine, or repeating undershooting and overshooting, that is, so-called hunting” by such control (JP-A-2006-71096, para. 0037).
In a case where a clutch shoe has been worn in a considerable degree due to aged deterioration and the like, the engine rotational speed becomes abnormally high at startup by the time the centrifugal clutch is engaged. As described above, in the case where clutch shoe 302 has been worn in a considerable degree due to aged deterioration and the like, a stalling state of a centrifugal clutch 300 continues for a long period of time, and a rotational speed of an engine 100 becomes abnormally high. Therefore, an abraded member of clutch shoe 302 comes in a state where they can be further easily worn. As a result, replacement of clutch shoe 302 is required at a faster pace. Because change gear ratios are uniformly controlled to a predetermined LOW ratio in response to a throttle opening in the control of JP-A-3194641, the aforementioned problem is not solved by the control of JP-A-3194641.